Just to be sure, you used the open to disengage at least one friendly unit correct? There is a "and then" clause which states that you are required to meet the first clause to activate the second part of the ability.

There is no selection process, the system checks whether you have a unit that's engaged or not and disengages if it found any. If you do not disengage any engaged friendly on your side of the field, you will not engage the enemy with this skill.

So, did you have an engaged friendly unit on your side of the field or not when this skill activated?